A radiation imaging apparatus includes, for example, a plurality of sensors arrayed so as to form a plurality of rows and a plurality of columns, a drive unit which drives each sensor row by row, and a readout unit which reads out a signal from each driven sensor.
In Japanese Patent Laid-Open No. 2013-98796, the start of radiation irradiation is detected based on signals from some of the plurality of sensors and the end of radiation irradiation is detected based on the signals from the some sensors in the radiation imaging apparatus. According to Japanese Patent Laid-Open No. 2013-98796, signals are read out from some of the sensors while maintaining the operation mode of the readout unit to have a mode with a high signal amplification ratio in order to improve the detection accuracy of the start of radiation irradiation. Then, after the start of radiation irradiation is detected, the operation mode of the readout unit is changed to a mode with a low signal readout ratio to prevent the saturation of the signals that have been read out from some of the sensors.
Some radiation imaging apparatuses can monitor the radiation dose and cause radiation irradiation to end (for example, output, to the radiation source, a signal to stop radiation irradiation) when the dose reaches a target value. This operation is called Automatic Exposure Control (AEC) and can, for example, prevent excessive radiation irradiation.
In Japanese Patent Laid-Open No. 2010-75556, some sensors are used as monitoring sensors to monitor the radiation dose, the signals of the monitoring sensors are monitored after the start of radiation irradiation, and radiation irradiation is ended based on the monitoring result. Additionally, in Japanese Patent Laid-Open No. 2010-75556, the signals of the monitoring sensors are monitored at least twice, and the timing to end radiation irradiation is determined based on these timings and theses monitoring results.
Generally, when the intensity of radiation is comparatively high (or low), the time until the radiation dose can reach the target value can be considered to become short (or long) and requires AEC of a given intensity of radiation to be properly performed in a radiation imaging apparatus. However, in a case in which the intensity of radiation is higher than an assumed value or in a case in which the signal amplification ratio of the readout unit is not properly set, the output value of the readout unit may become saturated (may exceed the output dynamic range).
According to the AEC method of Japanese Patent Laid-Open No. 2010-75556, radiation irradiation cannot be properly ended or the timing to end the radiation irradiation cannot be properly determined when the output value (monitoring result) of the readout unit becomes saturated. As a result, the accuracy of AEC is decreased. Also, if the signal amplification ratio of the readout unit is changed in accordance with the method of Japanese Patent Laid-Open No. 2013-98796 while AEC is being performed, the output value of the readout unit is changed by the change. Therefore, radiation irradiation cannot be properly ended or the timing to end the radiation irradiation cannot be properly determined, and as a result the accuracy of AEC is decreased even by the method of Japanese Patent Laid-Open No. 2013-98796.